In a reactor for a nuclear plant of the type defined above, a large number of elongated fuel units are arranged in the core of the reactor. Each fuel unit includes a number of elongated fuel rods. Each fuel rod includes an elongated cladding tube and a number of fuel pellets, which are provided in a pile in the cladding tube. The fuel rods in the fuel unit are maintained by means of a number of spacers, for instance 6-10 spacers, which are distributed along the length of the fuel unit. Each spacer defines cells for receiving the fuel rods. The spacers thus hold the fuel rods in a correct position in the fuel unit and have the function to ensure the maintaining of a constant mutual distance between the fuel rods during the operation of the reactor.
In a boiling water reactor, the fuel rods are normally enclosed in casings, so called boxes. Each box includes a relatively large number of fuel rods and forms together with these fuel rods a so-called fuel assembly, which can be lifted into and out of the core of the reactor. Each fuel assembly may include one or several fuel units. JP-7225291 discloses a fuel assembly having one such fuel unit. U.S. Pat. No. 5,875,223 discloses a fuel assembly having four such fuel units.
The core is submerged in a coolant, normally water, which functions both as coolant and as moderator. The fuel units and fuel rods are normally provided substantially vertically in the reactor. The coolant normally flows from below and upwardly. It is important to maintain a proper cooling of the fuel rods in the reactor. In a boiling water reactor it is especially critical to obtain a proper cooling in the upper part of the fuel rods where a significant part of the coolant (water) has been converted to steam. In the upper part of the fuel assembly, the coolant thus prevails in a two-phase state, wherein the liquid state partly flows as a film on the different parts of the fuel assembly, inter alia the surfaces of the fuel rods, the spacers and the inner side of the casing, and partly as droplets in the steam flow. If the coolant film on the surfaces of the fuel rods is not maintained an isolating steam layer is formed on the fuel rod leading to a quick temperature increase, so called dry-out, which can lead to defects of the cladding tubes.
The design of the spacers influences the flowing of the coolant and thus the cooling of the fuel rods. It is known to provide the spacers with deflection members provided for deflecting the coolant towards the fuel rods. Such deflecting members have the disadvantage that they, if they are used to a too large extent, result in a substantial increase of the pressure drop coefficient of the spacer. The percentage of steam is highest in the upper part of the fuel assembly. Due to the high percentage steam in the upper part of the fuel assembly, the pressure drop frequently is higher in this part than in the lower part of the fuel assembly. The larger the difference in pressure drop between the upper part of the fuel assembly and lower part, the higher the risk that the core becomes unstable. In order to give the fuel assembly proper stability properties, it is aimed at a low-pressure drop in the upper part of the fuel assembly.
There are spacers of a plurality of different types, for instance spacers formed by crossing sheets, spacers where the cells are formed by open elements having support points and spring members and spacers formed by sleeve-like members welded together. The spacers used today are normally manufactured of zirconium-based alloys (Zircaloy), nickel-based alloys (Inconel), combinations of these alloys or stainless steel. The present invention refers to a spacer formed by sleeve-like members.
A spacer of the kind initially defined is disclosed in U.S. Pat. No. 5,875,223. The known spacer thus includes welded sleeves forming the cells mentioned above. Each of the sleeves has a lower edge and an upper edge. The upper edge is parallel to a plane whereas the lower edge has a wave-shape with wave peaks and wave valleys. The purpose of this design of the lower edge is to prevent possible debris particles in the coolant from getting caught in the spacer, and thus to reduce the wear of the fuel rods.
JP-6-148370 discloses a sleeve spacer for a boiling water reactor. Each sleeve has inwardly directed projections for abutting the fuel rod extending through the sleeve. The projections extend merely over a small part of the length of the sleeve. Each sleeve is also according to one example at the lower end provided with a bevel. According to another example, each sleeve has a wave shape at the lower end of the sleeve.
JP-7-225291 discloses another sleeve spacer for a boiling water reactor. The circular cylindrical sleeves are here provided with an upper, downstream end, which has triangular or rectangular projections extending upwardly. The lower end of the sleeve appears to be straight. Each sleeve may also include inwardly directed projections, which extend over merely a part of the length of the sleeve for abutting the fuel rod extending through the sleeve.
U.S. Pat. No. 5,331,679 discloses a further variant of a sleeve spacer having substantially circular cylindrical sleeves. The spacer is kept together by means of a band extending around the outer periphery of the spacer. Each sleeve has relatively short inwardly directed projections, which together with a spring element form abutment points against the fuel rod extending through the sleeve. Both the lower edge and the upper edge may according to one embodiment have a wave-like shape with wave peaks and wave valleys. The wave peaks of the upper edge appear to be aligned to a respective wave valley of the lower edge.
When designing a spacer consideration has to be taken to a plurality of different requirements, which at least partly are contradictory.    1) The spacer shall be sufficiently mechanically strong to reduce the bending and vibration of the fuel rods and to resist large thermal and hydraulic forces also at dimensioning events such as plant accidents and earthquakes.    2) The spacer has to be able to resist axial and radial dimension changes of the fuel rods.    3) The spacer has to give sufficient abutment surface to the fuel rods for minimising local wear and the risk for defects of the fuel rods.    4) The spacer shall be provided with a minimal amount of material for minimising the neutron absorption.    5) The spacer shall be designed to give a minimal flow resistance and thus a small pressure drop.    6) The spacer shall be designed in such a way that possible debris particles in the coolant do not get caught in the spacer in such a way that these debris particles can subject the fuel rods to wear.    7) The spacer shall be designed in such a way that it provides a proper cooling of the fuel rods through a suitable mixing of the coolant.    8) The spacer shall be manufactured in a relatively easy and inexpensive manner.